Aluminum housings that are adapted to support drive shafts are commonly used in the outboard industry. These housings are generally made by the die casting method, which, depending on the condition of the mold, can sometimes impart a rough exterior surface on the part of sharp corners. In operation as part of an outboard motor, these housings must have a smooth exterior surface in order to be hydrodynamicly efficient as well as aesthetically pleasing. In addition, the housing must not contain any sharp corners. Preferably, the housing should have small radii and blends at all corners in order to enhance the thickness of and ability for paint to adhere entirely to the housing.
It has been found that one way to enhance the smoothness of the surface and make small blended radii on all sharp corners is to drag finish the housing. Drag finishing is a common method of finishing certain aluminum and stainless steel parts. As its name implies, the part to be finished is dragged through a media of abrasive particles. The abrasion of the particles as they pass over the part act to smooth inconsistencies in the exterior surface of the housing and impart radii on edges.
For example, propellers have been made by a drag finishing process by simultaneously dragging and spinning the propellers within the medium. It is typical in drag finishing propellers to drag a set of propellers by revolving the set around a central spindle with each propeller attached to the bottom end of a vertical shaft which also rotates as the main spindle revolves. This type of operation is not conducive for use with certain housings, as it does not allow for even abrasion over the entire housing, especially the top, since the top is near the surface of the media while being dragged.